The insulin receptor (IR) and insulin-like growth factor receptor (IGF1R) family is a therapeutic target in oncology. Development of pharmaceuticals that target the IR/IGF1R receptor family has been based on observations suggesting that circulating levels or insulin and/or insulin-like growth factor 1 (IGF-1) are related to cancer ris. In addition, increases in either receptor, present both as IR-IR and IGF1R-IGF1R homodimers and as IGF1R-IR heterodimers may reduce relapse-free survival in breast cancer patients. However, simple increases in the total expression of IGF1R and IR monomers do not adequately explain the growth-promoting effects of IGF-1 on tumor cells. Rather, it seems that distribution of these receptors among homodimers and heterodimers regulates tumor cell responses to insulin and IGF-1 including cell proliferation and migratory potential. There is currently no direct method to evaluate the concentrations of these individual receptor species in breast cancer cells or, for that matter, in any other normal or cancerous tissue. Fluorescence correlation spectroscopy (FCS) and photon counting histogram (PCH) methods, to be implemented in a newly-developed 4-detector fluorescence correlation system, provide a path to such direct measurements and make possible the project's three specific aims: Aim 1: To develop new FCS and PCH methods to directly evaluate, on viable single cells, the actual surface concentrations of IR homodimers, IGF1R homodimers and IR-IGF1R heterodimers. Initial experiments will involve a model system consisting of CHO cells simultaneously expressing green and red fluorescent forms of IR and IGF1R (IR-GFP and IGF1R-mCherry), each expressed at varying levels. Aim 2: To evaluate the thermodynamics of IR and IGF1R homodimer and IR-IGF1R hybrid receptor formation. The goal of this aim is interpret FCS and PCH measurements of separate receptor species in terms of a quantitative model involving association constants for receptor pairs and receptor monomer expression levels. Aim 3: To evaluate receptor dimer species of, and growth-promoting effects of insulin or IGF-1 treatment on, breast cancer cells, including ones where the numbers of IR and IGF1R expressed have been artificially manipulated. In this aim correlation of hormone effects with measured levels of the various receptor species will be explored. Overall, this project will provide improved tools and data for quantitative understanding of how IR and IGF1R expression jointly regulate signaling by insulin and IGF-1 in particular cells. These results will facilitate understanding of how hormone responses can vary among cell types and lead ultimately to improved patient care through the use of targeted pharmaceutical agents that appropriately modulate hormone-mediated tumor cell proliferation.